FXUS65 KSLC 220355
AFDSLC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
855 PM MST MON DEC 21 2009

.SYNOPSIS...A TROUGH OVER THE PACIFIC NORTHWEST WILL DROP SOUTH
THROUGH THE GREAT BASIN AND INTO ARIZONA THE NEXT TWO DAYS. THE
BRUNT OF THE STORM WILL IMPACT CENTRAL AND SOUTHERN UTAH.

&&

.DISCUSSION...BIG PICTURE...A COLD UPPER TROUGH OVER THE PACIFIC
NORTHWEST TONIGHT WILL DROP SOUTH INTO THE GREAT BASIN OVERNIGHT
WITH THE TRACK OF THE COLD UPPER CORE GOING THROUGH NV AND INTO AZ
WITH THE COLDEST AIR ALOFT NEVER REALLY ENTERING UTAH. AS THIS
UPPER SYSTEM PULLS OFF TO THE EAST AN EVEN COLDER AIRMASS DROPS
INTO THE STATE FROM THE NORTH WED NIGHT.

INVERSION...NOT MUCH FLOW AT THE SURFACE AHEAD OF THE TROUGH SO IT
SEEMS UNLIKELY INVERSION WILL BE MIXED OUT BEFORE PRECIP BEGINS.
SO VARIOUS UTAH VALLEYS CURRENTLY SUBMERGED IN GREY
UNPLEASANTNESS LIKELY TO SEE DEEPER SHADES OF GREY TUESDAY AS
CLOUDS ABOVE THE INVERSION THICKEN AND EVENTUALLY PRODUCE SNOW
DURING THE DAY. WHEN THE PRECIP FINALLY ENDS LATE WED AND COLDER
AIR ALOFT MOVES IN FROM THE NORTH THE INVERSION SHOULD HAVE BEEN
DESTROYED ALTHOUGH MODEL SOUNDINGS INDICATE STABLE LAYER REMAINS ABOUT
12K MSL...BUT THIS SHOULD STILL DO THE TRICK. STRONGEST WINDS AT ABOUT
800 MB EXPECTED MID DAY TUESDAY...SO IF WE ARE GOING TO MIX THAT
WILL BE THE TIME...THEN FLOW ALOFT BECOMES VERY LIGHT AS UPPER LOW
FORMS TO THE SOUTH.

TROUGH AND SNOW...LATEST 00Z NAM IS DRIER OVER ALMOST THE ENTIRE
CWA COMPARED TO PREVIOUS RUNS WITH THE EXCEPTION OF THE LAKE
POWELL AREA WHERE MODEL CONVECTIVE PRECIP GOES WILD...WHICH DOES
NOT LOOK REASONABLE. THIS IS NOT A GREAT LOOKING STORM...THE BEST
PART OF IT IS HEADING FOR SOUTHERN UT WHERE AIRMASS BECOMES
UNSTABLE AND DYNAMIC LIFT WILL BE THE STRONGEST...WARNINGS ARE
CURRENTLY IN EFFECT BUT IF LATEST NAM IS CORRECT IT WILL BE A LOW
END WARNING AT BEST. IN THE NORTH....THE COLD FRONT ASSOCIATED
WITH THIS TROUGH WILL BE HARD TO IDENTIFY AND TIME/HEIGHT SECTIONS
AT SLC SHOW MOST OF THE PRECIP FALLS WITHIN A VERY STABLE AIRMASS
WITH MODEST WARM ADVECTION ALOFT IN THE WRAP AROUND PORTION OF THE
STORM. LOTS OF MOISTURE BUT NOT MUCH LIFT AND LITTLE FLOW FOR
OROGRAPHICS...ALL POINTS TO 24 TO 30 HOURS OF LIGHT PRECIP IN THE
NORTH. 00Z NAM WOULD INDICATE CURRENT WINTER STORM WATCH SHOULD
BECOME A SNOW ADVISORY WITH NO PART OF THE CURRENT WATCH LIKELY TO
SEE WARNING LEVEL SNOW. BUT WILL KEEP WATCH GOING FOR NOW AND LET
MID SHIFT DECIDE AFTER OTHER 00Z MODELS ARRIVE.

ONE LAST FORECAST PROBLEM WITH THIS TROUGH IS 00Z NAM BUFR
SOUNDINGS INDICATE AIRMASS WILL WARM JUST ENOUGH FOR PRECIP TO
TURN TO RAIN ALONG LOWEST ELEVATIONS OF WASATCH FRONT ON TUESDAY
AFTERNOON. THIS WOULD BE WELCOME BY RESIDENTS AS THE ONLY WAY THIS
COULD HAPPEN IS IF THE INVERSION IS WEAKENED
SIGNIFICANTLY...DIFFICULT TO SEE THIS HAPPENING UNLESS
AFOREMENTIONED 800 MB WINDS DUE THE JOB. AT THIS TIME IT APPEARS
IT WOULD EITHER BE RAIN OR SNOW...AND NOT FREEZING RAIN AS WARMING
IN BUFR SOUNDINGS IS AT THE SURFACE. IF INVERSION DOES NOT BREAK
AT ALL TUESDAY...EVEN LIGHT SNOW WILL STICK TO ROAD SURFACES AND
HAVE A MAJOR IMPACT ON TRANSPORTATION. IF INVERSION WEAKENS AND
SOME WARMING OCCURS...IMPACT OF LIGHT SNOW LIKELY TO BE QUITE A
BIT LESS.

&&

.AVIATION...CIGS AT THE SLC TERMINAL ARE EXPECTED TO BOUNCE BETWEEN 
LIFR AND IFR THROUGH BETWEEN 13Z AND 15Z. VSBYS WILL BE 
PREDOMINANTLY IN THE IFR TO LOW END MVFR RANGE DURING THIS SAME TIME 
PERIOD...BUT MAY BECOME LIFR AT TIMES IN FOG. WINDS ARE EXPECTED TO 
BE SOUTHERLY BY 5Z...BUT WILL BE LIGHT AND VARIABLE AT TIMES. 

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...WINTER STORM WARNING FROM MIDNIGHT TUESDAY TO 11 PM WEDNESDAY 
     FOR UTZ014-UTZ517-UTZ518.

     WINTER STORM WARNING FROM 3 AM TUESDAY TO 5 PM WEDNESDAY FOR 
     UTZ005-UTZ015-UTZ016.

     WINTER STORM WATCH FROM 5 AM TUESDAY TO 5 PM WEDNESDAY FOR 
     UTZ003-UTZ004.

     WINTER STORM WATCH FROM 5 AM TUESDAY TO 11 PM WEDNESDAY FOR 
     UTZ008-UTZ009-UTZ010.

WY...NONE.

&&

$$

PUBLIC...DUNN
AVIATION...HOSENFELD

FOR MORE INFORMATION FROM NOAA/S NATIONAL WEATHER SERVICE VISIT...
HTTP://WEATHER.GOV/SALTLAKECITY (ALL LOWER CASE)